BACKGROUND: Identifying how greenspace impacts the temperature-mortality relationship in urban environments is crucial, especially given climate change and rapid urbanization. However, the effect modification of greenspace on heat-related mortality has been typically focused on a localized area or single country. This study examined the heat-mortality relationship among different greenspace levels in a global setting. METHODS: We collected daily ambient temperature and mortality data for 452 locations in 24 countries and used Enhanced Vegetation Index (EVI) as the greenspace measurement. We used distributed lag non-linear model to estimate the heat-mortality relationship in each city and the estimates were pooled adjusting for city-specific average temperature, city-specific temperature range, city-specific population density, and gross domestic product (GDP). The effect modification of greenspace was evaluated by comparing the heat-related mortality risk for different greenspace groups (low, medium, and high), which were divided into terciles among 452 locations. FINDINGS: Cities with high greenspace value had the lowest heat-mortality relative risk of 1·19 (95% CI: 1·13, 1·25), while the heat-related relative risk was 1·46 (95% CI: 1·31, 1·62) for cities with low greenspace when comparing the 99th temperature and the minimum mortality temperature. A 20% increase of greenspace is associated with a 9·02% (95% CI: 8·88, 9·16) decrease in the heat-related attributable fraction, and if this association is causal (which is not within the scope of this study to assess), such a reduction could save approximately 933 excess deaths per year in 24 countries. INTERPRETATION: Our findings can inform communities on the potential health benefits of greenspaces in the urban environment and mitigation measures regarding the impacts of climate change. FUNDING: This publication was developed under Assistance Agreement No. RD83587101 awarded by the U.S. Environmental Protection Agency to Yale University. It has not been formally reviewed by EPA. The views expressed in this document are solely those of the authors and do not necessarily reflect those of the Agency. EPA does not endorse any products or commercial services mentioned in this publication. Research reported in this publication was also supported by the National Institute on Minority Health and Health Disparities of the National Institutes of Health under Award Number R01MD012769. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health. Also, this work has been supported by the National Research Foundation of Korea (2021R1A6A3A03038675), Medical Research Council-UK (MR/V034162/1 and MR/R013349/1), Natural Environment Research Council UK (Grant ID: NE/R009384/1), Academy of Finland (Grant ID: 310372), European Union's Horizon 2020 Project Exhaustion (Grant ID: 820655 and 874990), Czech Science Foundation (22-24920S), Emory University's NIEHS-funded HERCULES Center (Grant ID: P30ES019776), and Grant CEX2018-000794-S funded by MCIN/AEI/ 10.13039/501100011033 The funders had no role in the design, data collection, analysis, interpretation of results, manuscript writing, or decision to publication.

Center for Environmental and Respiratory Health Research University of Oulu Oulu Finland

Department of Earth Sciences University of Torino Turin Italy

Department of Environmental Health French National Public Health Agency Public Health France Saint Maurice France

Department of Environmental Health Harvard T H Chan School of Public Health Boston MA USA

Department of Environmental Health National Institute of Health Dr Ricardo Jorge Lisbon Portugal; EPI Unit Institute of Public Health University of Porto Lisbon Portugal

Department of Environmental Health National Institute of Public Health Cuernavaca Morelos Mexico

Department of Epidemiology and Preventive Medicine School of Public Health and Preventive Medicine Monash University Melbourne VIC Australia

Department of Family Medicine and Public Health University of Tartu Tartu Estonia

Department of Geography University of Santiago de Compostela Santiago de Compostela Spain

Department of Hygiene Epidemiology and Medical Statistics National and Kapodistrian University of Athens Greece

Department of Public Health and Clinical Medicine Umeå University Sweden

Department of Public Health Environments and Society London School of Hygiene and Tropical Medicine London UK

Department of Public Health Environments and Society London School of Hygiene and Tropical Medicine London UK; Centre for Statistical Methodology London School of Hygiene and Tropical Medicine London UK; Centre on Climate Change and Planetary Health London School of Hygiene and Tropical Medicine London UK

Department of Statistics and Computational Research Universitat de València València Spain

Department of Statistics Computer Science and Applications G Parenti University of Florence Florence Italy

Faculty of Geography and Environmental Sciences Hakim Sabzevari University Sabzevar Khorasan Razavi Iran

Gangarosa Department of Environmental Health Rollins School of Public Health Emory University Atlanta GA USA

Graduate School of Public Health Seoul National University Seoul Republic of Korea

Institute of Advanced Studies University of São Paulo São Paulo Brazil

Institute of Atmospheric Physics Czech Academy of Sciences Prague Czech Republic; Faculty of Environmental Sciences Czech University of Life Sciences Prague Czech Republic

Institute of Atmospheric Physics Czech Academy of Sciences Prague Czech Republic; Faculty of Environmental Sciences Czech University of Life Sciences Prague Czech Republic; Global Change Research Institute Czech Academy of Sciences Brno Czech Republic

Institute of Environmental Assessment and Water Research Barcelona Spain; School of Tropical Medicine and Global Health Nagasaki University Nagasaki Japan

Institute of Social and Preventive Medicine University of Bern Bern Switzerland

National Taiwan University and National Taiwan University Hospital Taipei Taiwan; National Institute of Environmental Health Science National Health Research Institutes Zhunan Taiwan

School of Epidemiology and Public Health Faculty of Medicine University of Ottawa Ottawa ON Canada; Air Health Science Division Health Canada Ottawa ON Canada

School of the Environment Yale University New Haven CT USA

School of Tropical Medicine and Global Health Nagasaki University Nagasaki Japan

Shanghai Children's Medical Centre Shanghai Jiao Tong University School of Medicine Shanghai China; School of Public Health Institute of Environment and Population Health Anhui Medical University Hefei China; Center for Global Health School of Public Health Nanjing Medical University Nanjing China; School of Public Health and Social Work Queensland University of Technology Brisbane Australia

Swiss Tropical and Public Health Institute Basel Switzerland; University of Basel Basel Switzerland

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